Process for the continuous production of thin-walled hollow profiles which are composed of nonferrous metals and have small diameters and are corrugated in sections

ABSTRACT

A process for the continuous production of thin-walled, radially closed hollow profiles which are composed of nonferrous metals and have a small cross section comprises supply of a flat strip of the nonferrous metal to a forming apparatus ( 212 ) at a first supply speed, where the thickness of the strip corresponds to the wall thickness of the hollow profile. The forming apparatus ( 212 ) is configured for continuous forming of the flat strip supplied into a shape corresponding to the hollow profile. After forming, two opposite edges of the flat strip rest flush against one another in a contact region. A welding apparatus ( 216 ) continuously welds the edges which rest flush against one another by means of a laser which emits light having a wavelength of less than 600 nm. The laser heats a point in a welding region which has a diameter which is less than 20% of the cross-sectional dimension of the hollow profile. The welded hollow profile is taken off from the welding region, provided in a corrugator ( 225 ) is with parallel or helical corrugation in sections and taken up in an uptake device ( 226 ).

RELATED APPLICATION

This application claims the benefit of priority from European PatentApplication No. 19 306 242.9, filed on Sep. 30, 2019, the entirety ofwhich is incorporated by reference.

FIELD OF THE INVENTION

The invention relates to the continuous production of tubes or hollowprofiles composed of nonferrous metals, in particular the continuousproduction of thin-walled tubes which have small diameters and arecorrugated in sections.

BACKGROUND

To produce piping systems, not only straight sections but also, interalia, curved sections are necessary. Usually, straight and curved tubesegments are welded abutting one another at the end faces along the runof the pipe, joined with a muff by pressing or soldering or joined inanother way.

Corrugated tube segments are required for some applications, for examplein heat exchangers where enlargement of the surface area is achieved bymeans of the corrugation or when a tube segment is to be more easilybent away from its longitudinal axis without the bending resulting in anundesirable deformation of the tube cross section. Corrugated tubesegments are also frequently used when length changes caused by thermalexpansion or mechanical loads have to be compensated for.

Corrugated tubes can, for example, be produced from smooth-walled tubeswhich are provided with parallel or helical corrugation in a corrugator.When corrugated tube segments are to be joined to smooth-walled tubesections, the ends of the tubes to be connected have to be smooth-walledand have a matching diameter in order to make connection possible. Inthe case of corrugated tube segments, the ends thus have to be rolledagain to give a smooth-walled round tube without corrugation in acomplicated forming process in order for tube sections or flanges to beable to be welded on. Joining of the tube segments then occurs by meansof welding, for example tungsten-inert gas welding (WIG) in a seam whichruns over the circumference of the join. As an alternative, joining canalso be effected, depending on the application, by means of shrinktubes, muffs, sleeve nuts, cutting ring screw connections, etc. It iscommon to all these connection variants for long tubes firstly to bedivided into smaller pieces which subsequently have to be aligned withone another and joined together in a more or less complicated fashion.

In the case of heat exchangers, thin-walled tubes having small diametersso as to give good heat exchange are of particularly great interest, notleast because a relatively large number of tubes having small diameterscan be arranged in a fixed volume of a heat exchanger and a larger areafor heat exchange is thus available.

In the conventional production of thin-walled hollow profiles or tubeshaving small diameters in a continuous process, a slitted hollow profilehaving larger dimensions, for example a slitted tube having a largerdiameter, and optionally a greater wall thickness is firstly made from aflat metal strip, for example by means of appropriate rollers andprofiles. The edges adjoining the slit of the metal strip which has beenshaped to give the hollow profile are subsequently welded together,usually employing an electric arc welding process. Electric arc weldingproduces a pronounced welding bead on the inside of the tube, projectinginto the interior of the tube. For this reason, the welding seam has tobe smoothed or deburred on the outside and/or inside after welding, orthe bead is removed by cutting machining, before the tube can beprocessed further. To ensure that the welding seam is uniform, it ispossible to carry out a series of non-destructive tests, e.g. eddycurrent testing, ultrasonic testing and pressure tests.

Exactly as in the production of seamless tubes having a small diameterand/or small wall thicknesses, the welded tubes are subsequentlysubjected to a drawing process in which the wall thickness is reduced bymeans of a mandrel, plug or a spike in order to attain a smallerdiameter and optionally lower wall thicknesses of the end product. Inparticular because of the large dimensions of the welding seam in theelectric arc welding process and the resulting large minimum diameterwhich the welded tube has to have, a number of drawing processes withincreasing degree of deformation may be necessary in order to achievesmall cross sections and thinner wall thicknesses. Tube diameters below4 mm can no longer be produced reliably by means of electric arcwelding. If only the wall thickness has firstly been reduced in a firstdrawing process, the diameter of the tube subsequently has to bedecreased. For this purpose, the tube can, for example, be drawn over afloating plug and through a drawing die. However, the reduction in thewall thickness and the diameter can also be effected in a single drawingoperation. During drawing over a plug, not only is the wall thickness orthe diameter reduced but the welding bead on the inside is alsosmoothed. A further reduction in the diameter to the desired finaldimension can be achieved in one or more subsequent drawing processes,in which the plug can be dispensed with when no further decrease in thewall thickness is required. FIG. 4 shows a schematic depiction of theelements used in plug drawing. In plug drawing, a plug 3 is positionedin the interior of the tube 1 which is then drawn through an opening ina die 2 having a smaller diameter than that of the original tube andthat of the plug. The plug cannot be drawn through the tube and movesrelative to the tube; in actual fact it remains in front of the openingwhile the tube is pulled through between the opening and the plug andits diameter and its wall thickness are reduced in the process. In thisoperation, the tube can be preheated in addition to heating broughtabout by the deformation in order to increase the ductility of thematerial during drawing. A drawing agent which reduces the friction ofthe plug can be introduced into the interior of the tube. The processcan be repeated a number of times with decreasing diameters. When thework hardening of the material exceeds the desired degree or the degreewhich the material can tolerate, an annealing step in which the drawntubes are, before a subsequent drawing operation, degreased, heattreated and descaled and sharpened for the next drawing operation can beintroduced. The annealing homogenizes the metallurgical microstructureof the tube. Since the microstructure of a tube has been destroyed afterdrawing, there is, in particular, accumulation of dislocations and thematerial becomes hard and brittle: this is referred to as work hardeningor strain hardening. In order to be able to draw the drawn tubes again,the dislocations in the microstructure have to be eliminated and theresidual stresses have to be dissipated so that the material is broughtback to its original state. During annealing, the drawn tubes aresubjected to a controlled temperature, up to 1200° C. depending on thematerial, and an annealing time. During this operation, the tube usuallyretains its shape, at least in the case of simple cross sections, butthe grains in the microstructure of the tube form a normal unstressedpattern again. Depending on annealing temperature, residual stresses andmicrostructural defects such as dislocations are dissipated by crystalrecovery, i.e. movement of dislocations, and recrystallization. Theannealed tube is then softer and can be drawn again. Depending on thedesired target hardness, a concluding heat treatment is not necessaryfor high-pressure tubes. The material thus remains in the hard-drawnstate and in this way improves the mechanical properties of the tube.Both welded tubes and long seamless tubes can in this way be drawn to anannular tube shape and very long annular tubes produced.

For particular applications, tubes composed of nonferrous metals areparticularly suitable. Copper or aluminium tubes can, for example, bepreferably employed in heat exchangers because of the high thermalconductivity. In heat exchangers, tubes having small diameters and lowwall thicknesses are required, firstly to hinder heat transfer betweenthe media as little as possible and keep the heat capacity of the tubeitself small, and secondly to keep material usage and weight low. Wallthicknesses less than 0.15 mm in particular can no longer be producedreliably and with the required quality of the welding seam by means ofelectric arc welding. Tubes composed of nonferrous metals having wallthicknesses and diameters smaller than those mentioned accordingly mustbe brought to the respective desired final dimension by means ofprocessing steps following the actual production of the tube.

It is fundamentally advantageous for individual tubes to be able, wherepossible, to be produced as long parts in order to allow a largelycontinuous production process which has to be interrupted as rarely aspossible. Any subpieces required can then be cut from the long tubes,with only a small cutting residue, if any, remaining. In general, anysaving on process steps is naturally advantageous in the productionprocess.

It is an object of the present invention to propose an apparatus and aprocess by means of which thin-walled tubes or hollow profiles can beproduced in a continuous process and selectively provided in sectionswith helical or parallel corrugation.

SUMMARY OF THE INVENTION:

This object is achieved by the process indicated in claim 1 and theapparatus indicated in claim 8. Further developments and embodiments arein each case indicated in the dependent claims.

In the process of the invention for the continuous production ofthin-walled, radially closed hollow profiles which are corrugated insections, a flat strip composed of a nonferrous metal, whose thicknesscorresponds to the wall thickness of the hollow profile to be produced,is firstly supplied. The width of the metal strip supplied preferablycorresponds to the circumference of the hollow profile. If the metalstrip supplied is wider than that required for the circumference of thehollow profile, or if the edges of the metal strip are not sufficientlysmooth, the metal strip can be cut to size on one or two sides in acontinuous process while being supplied. In the present description, theexpression “thin-walled” refers to wall thicknesses of a few tenths of amillimetre, in particular less than 0.15 mm. The term nonferrous metalis, for the purposes of the present description, used both for themetals themselves and for alloys thereof.

The metal strip present in the appropriate width is shaped in asingle-stage or multistage continuous forming process to give a hollowprofile which has the desired cross section. The forming process cancomprise successive bending in the longitudinal direction of the stripin a plurality of stages, for example on appropriately designed rollersand profiles. After forming, the hollow profile has a region which runsin the longitudinal direction of the hollow profile and in which theedges of the metal strip adjoin one another in an abutting fashion. Theabutting edges of the hollow profile are then welded to one anotheralong the butt seam. According to the invention, welding is carried outusing a laser which radiates light having a wavelength of less than 600nm, preferably in the range from 550 to 450 nm. Wavelengths in a rangebelow 450 nm can also be advantageously used for the purposes of theinvention. The laser introduces light energy into a point in the weldingregion, which light energy is, on impingement on the surface of thematerial being welded, absorbed and converted into heat. Light in thespecified wavelength ranges utilized according to the invention isabsorbed very much better by many nonferrous metals even at roomtemperature than, for example, light in the infrared spectrum havingwavelengths above about 800 nm. In actual fact, light is absorbed atwavelengths above about 600 nm so poorly by many nonferrous metals thatlasers having particularly high output powers and particular coolingmeasures would be necessary in order to weld the nonferrous metal. Inaddition, the absorption at wavelengths above 600 nm is greatlydependent on the nature of the surface, while the influence of thenature of the surface decreases greatly at the wavelengths utilizedaccording to the invention. Furthermore, rapid regulation of the energyintroduced into the active welding region is necessary because of thehigh temperature dependence of the absorption at relatively highwavelengths in particular, which is virtually impossible to implement,so that the quality of the welding seam can fluctuate greatly. The useaccording to the invention of light having wavelengths of less than 600nm produces a more stable melt bath and leads to an overall more stableprocess which gives, at a high energetic efficiency of the weldingprocess, longitudinally welded hollow profiles having a high quality andproduces less reject material. In addition, a pretreatment of thewelding region which brings about a reduction in the reflection and thusan increase in the absorption of the laser light can be dispensed withat the wavelength below 600 nm as is utilized according to theinvention. The welding region thus does not have to be, for example,roughened or preheated, and it is not necessary to apply any layer of amaterial which acts as “mediator” to convert the irradiated light energyinto heat and transfer it to the material being welded, so that itstemperature-dependent degree of absorption is moved into regions morefavourable for the wavelength used, in the welding region. Thiseliminates the risk of parts of the material used as mediator gettinginto the welding seam.

The absorbed light brings about strong heating of the metal. In order tointroduce sufficient energy into the material to be welded, the lighthas to be highly focused. A high degree of focusing is also necessarybecause welding is to be effected only in the contact region of theedges along the slit. Due to heat conduction within the nonferrousmetal, regions directly adjoining the point of impingement of the laserbeam can likewise be strongly heated and possibly melt. Especially atsmall cross-sectional dimensions of the hollow profiles to be produced,for example at diameters of less than 4 mm, focusing of the laser beamis therefore of great importance in order to avoid uncontrolledflowing-away of liquefied material or detachment of material. In theprocess of the invention, the laser beam has a diameter at the workpieceof not more than 20% of the cross-sectional dimensions of the hollowprofile, preferably less than 10%. Experiments have shown that diametersof the laser beam down to 5% of the cross-sectional dimensions stillmake it possible to produce welding seams having good quality, but inthis case further measures may be necessary, for example movement of thefocus point over the welding region. In the case of a hollow profilehaving a diameter of 4 mm, the diameter of the laser beam canaccordingly be, for example, 400 μm, preferably 200 μm or less.

The high local energy density at the point of impingement of the laserbeam on the workpiece brings about local melting of the material on bothsides of the butt seam, so that the melts flow into one another. Thematerial solidifies again when it is no longer struck by the laser beamand forms the welding seam. Since the hollow profile is continuouslyconveyed past the fixed laser, a continuous welding seam which joins thetwo edges is produced. In order to prevent uncontrolled flowing-away ofthe liquid material, which is present in a small wall thickness, thelaser power introduced and the speed at which the tube is conveyed pastthe laser have to be matched to one another. In the case of suitablematching, smooth welding seams which require no after-working areobtained on the outside and on the inside.

In contrast to the known electric arc welding by the tungsten-inert gasprocess (WIG) or metal-inert gas process (MIG), which prevent reactionof the melt with the ambient air by means of the inert gas atmosphereand therefore make high seam qualities possible, nonferrous metalshaving material thicknesses of less than 0.15 mm can be welded flushwith one another in such a way that no after-working of the weldingseam, in particular on the inside of the tube, is necessary even withoutprotective gases in the laser welding utilized according to theinvention because of the better controllability of the energy input. Inembodiments of the process, an inert protective gas, for example argon,can nevertheless be passed over the welding position or cover thewelding position on the inside and/or outside. Use of a protective gasatmosphere can be dependent on, inter alia, the material to be weldedand the thickness thereof.

Control of the energy input by the laser can be effected either byfocusing on a larger target region, so that energy available acts asrequired on a larger or smaller area, or by appropriate back and forthmovement of a particularly narrowly focused laser beam. Focusing over alarger target region can also be brought about by means of a laserprofile which has a central focus point of high intensity and an annularregion of lower intensity surrounding the central focus point. In thisway, the welding region can be heated or cooled along a temperatureprofile, which can give a cleaner welding seam, and the solidificationmicrostructure can be influenced in a targeted way. In addition, laserbeams can be pulsed in a simple way, with control of the energy inputoccurring, for example, via the pulse duration and the pulse spacing.

Welding by means of lasers, also known as heat conduction welding,produces a smooth, rounded welding seam which no longer has to beafter-worked. The energy becomes distributed outside the region in whichthe laser impinges into the workpiece only by conduction of heat in thecase of heat conduction welding. For this reason, the seam depth is,depending on the laser power and the thermal conductivity of thematerial, only from a few tenths of a millimetre to about 1 millimetre.The thermal conductivity of the material limits the maximum seam depth.In general, the width of the seam is greater than the depth of the seam.When the heat cannot flow away fast enough, the temperature of theregion being worked rises above the vaporization temperature, so thatmetal vapour arises and the welding depth increases sharply. The processthen goes over into deep welding.

After welding, the hollow profile is fed into a corrugator whichintroduces a helical or parallel corrugation into the hollow profile.The corrugator is designed for variable setting of the corrugating tool,so that corrugated tube sections can be introduced selectively, i.e.tube sections with corrugation and smooth tube sections alternate. Thismakes it possible to position corrugated sections along the tube in theregions where curved tube pieces are otherwise welded on. The greaterflexibility of the tube compared to a smooth tube achieved by means ofthe corrugation allows the required change in direction of the tube axisat the positions provided. Noncorrugated regions of the tube bringstiffness where it is required. They also assist, for example, theattachment of fastening elements or sealing against housing elements.

The high quality of the welding seam of the tube produced according tothe invention which has no pronounced bead of material along the weldingseam due to the finely controllable energy input into the weldingposition makes it possible to dispense with laborious after-workingbefore corrugation, which is especially difficult to implement in thecase of small tube diameters.

In one or more embodiments of the process, the width of the stripsupplied is measured and a cut width is provided as a function of themeasurement result and a prescribed value. The width correspondsapproximately to the circumference of the hollow profile along theneutral fibre. Here, the prescribed value can be varied and a formingapparatus can be controlled as a function of the varying width of thestrip, for example to adapt the amount of material required for thewelding seam.

In embodiments of the process, a temperature profile is measuredtransverse to the welding seam. The measured temperature profile can beutilized for controlling the energy introduced into the welding point.The measured temperature profile can, for example, be compared with aprescribed profile and the control of the energy introduced canencompass variation of the focus diameter, a locus described by thefocus point on the material being welded and/or a change in the pulseduration and/or the pulse spacing of the laser beam. It is likewiseconceivable to regulate the supply speed as a function of the measuredtemperature profile. The measured temperature profile can also be storedfor quality management and documentation purposes.

In embodiments of the process, the welding seam is checked by means ofultrasound, X-rays, an eddy current measurement or other non-destructivemeasurement methods. The results of the check can, for example, beutilized for controlling the energy introduced into the welding positionand/or the supply speed.

In embodiments of the process, a tensile force acting on the flat stripof nonferrous metal and/or on the welded hollow profile is determinedand drives which feed the flat strip to forming and/or welding and/orfeed the welded hollow profile to an uptake apparatus are regulated onthe basis of the previously determined tensile force. A tensile forcewhich is too high can, especially in the case of supplied strips havinga very small thickness, lead to tearing of the strip, which wouldinterrupt the process. An analogous situation applies to the tensileforce acting on the welded hollow profile.

An apparatus according to the invention for the continuous production ofthin-walled, radially closed hollow profiles which are composed ofnonferrous metals and are corrugated in sections comprises a feed deviceequipped for supplying a flat strip of the nonferrous metal. The feeddevice can, for example, comprise a holder for a flat strip of thenonferrous metal wound up on a reel or in the form of a coil. The stripis wound off from the reel and fed to a forming apparatus which formsthe flat strip of nonferrous metal into the profile of the hollowprofile so that the opposite edges of the flat strip of the nonferrousmetal abut one another in a flush manner. The forming apparatus can, forexample, comprise a plurality of rollers and profiles, for exampledrawing dies, which form the strip to give the desired hollow profileduring passage in the longitudinal direction. The forming apparatus canadditionally have two or more guide means which are at a distance fromone another in the longitudinal direction of the formed strip or hollowprofile, between which guide means the edges are held flush against oneanother at a position to be welded. The strip can optionally be guidedat the sides at one or more points before and in the tool in order tominimize sideways movement of the strip.

The apparatus further comprises a welding apparatus which welds togetherthe edges resting flush against one another between the guide means. Thewelding apparatus comprises a laser which emits light having awavelength of less than 600 nm with an energy which brings about localmelting of the nonferrous metal at both sides of the edges. As a resultof the continuous advance of the formed and welded hollow profile,regions in which the material has been melted are moved out from theregion in which the laser heats the material and the molten materialsolidifies again. The energy introduced into the material in order toheat it is matched to the material, the thickness thereof and also thespeed at which the hollow profile is conveyed past the welding position,so that although the material is melted in a region located directly atthe edges resting flush against one another, no liquid material runsinto the interior of the hollow profile. The distance between optics ofthe laser and the edges of the hollow profile to be welded can be keptconstant by means of the guide means. In order to keep the position ofthe adjacent edges constant relative to the optics of the laser, a“sword” can be arranged in the longitudinal slit located between theedges before the guide means which close the longitudinal slit in orderto prevent helical twisting.

The apparatus additionally comprises one or more transport devices whichconvey the welded hollow profile on to a corrugator which introduces ahelical or parallel corrugation into the hollow profile. The corrugatoris equipped for variable setting of the corrugating tool, so thatcorrugated tube sections can be introduced selectively, i.e. tubesections with corrugation and smooth tube sections alternate. The hollowprofile which has been corrugated in sections is then conveyed furtherto an uptake device which takes up the hollow profile. The transportdevice can comprise, for example, one or more clamping tong offtakes,cleat offtakes, disc offtakes or belt offtakes of known constructiontype, with different transport devices also being able to be combinedwith one another.

In one or more embodiments of the apparatus, a measurement apparatus fordetermining the tensile force is provided upstream of the formingapparatus. The tensile force determined can be supplied as actual valueto a regulator and used together with an intended value for regulatingthe drives of the apparatus, for instance in order to regulate the speedat which the strip of nonferrous metal is supplied. In addition, ameasurement and/or regulating apparatus can be arranged downstream ofthe welding device in order to measure the tensile force exerted on thewelded hollow profile and/or regulate the drive of the transport devicewhich feeds the welded hollow profile to the uptake device. Regulationof the tensile force between the transport device and the uptake devicecan, for example, be effected by means of a dancer which measures a sagof the welded hollow profile and feeds corresponding signals to a drivecontrol of the uptake device.

In one or more embodiments, the apparatus additionally comprises acutting device arranged upstream of the forming device, by means ofwhich one or both edges of the flat strip of nonferrous metal suppliedare cut, with the width of the cut strip corresponding to thecircumference of the hollow profile. In these embodiments, hollowprofiles having different circumferences can be produced without greatdifficulty by cutting the metal strip supplied to the required width andadapting the further tools of the apparatus.

Pieces cut off at one or both edges of the strip can, in one or moreembodiments, be fed to an apparatus for accommodating cutting scrap.

In one or more embodiments of the apparatus equipped with a cuttingdevice, a measurement device for measuring the width of the strip whichhas been cut to size is provided downstream of the cutting device. Thecutting device can be controlled with the aid of the measured values inorder to maintain a desired width of the nonferrous metal strip over along period of time. The cutting device can be supplied with appropriateprescribed values with which the measured width of the nonferrous metalstrip is compared in order to generate a control signal for adjustingthe cutting device. The width corresponds approximately to thecircumference of the hollow profile along the neutral fibre.

In one or more embodiments, the apparatus additionally comprises ameasurement device for determining a temperature profile transverse tothe welding seam. The measured temperature profile can be supplied tothe welding apparatus in order to control the energy given off, or tothe feed device and/or the transport device in order to control thesupply speed.

In one or more embodiments, the apparatus additionally comprises ameasurement device for measuring at least one dimension of the hollowprofile after welding. This measurement device can be used forintegrated quality control, exactly like a measurement device providedin one or more embodiments for checking the welding seam and/or materialdefects or inhomogeneities in the material. The dimensions canpreferably be measured in a contactless manner, for example by means oflasers.

The direct and continuous manufacture of the tube which has the desireddiameter and the desired low wall thickness and is corrugated insections advantageously decreases the additional production stepsrequired hitherto, which are necessary, for example, in the productionof tube systems having many changes in direction of the tubes, forinstance in the production of heat exchangers.

Corrugated regions arranged at fixed distances from one another alongthe hollow profile can be advantageous for producing tubes for heatexchangers. The smooth sections can be located in the heat exchanger,for example be welded to sheet-like elements or rest flush against themin a thermally coupled manner, which increase the effective surface areaof the heat exchanger, while the tubes in the corrugated regions arebent in order to allow a serpentine configuration of the tubes at or inthe sheet-like elements. Bending of, in particular, very thin-walledtubes frequently leads to undesirable kinks and the undesirable profilenarrowing associated therewith; in addition, the freedom from leaks of atube can no longer be ensured at the position of a kink. A supporting ofthe inside of the tube which is frequently utilized in the bending ofthin-walled tubes is not useable in the continuous production accordingto the invention of the tube. However, thin-walled tubes can be bent ina defined manner in the corrugated regions without changing the crosssection in an uncontrolled way. Since the tubes together with thecorrugated regions can be produced continuously in one productionoperation, the connection of straight tube sections to curved tubesections by means of which a change in direction is achieved, as hasbeen hitherto necessary, can be dispensed with.

The corrugation can optionally also be provided in straight sections,for example to influence flow conditions of a fluid flowing in thehollow profile and/or around the hollow profile or in order to increasethe surface area available for heat transfer.

Hollow profiles having wall thicknesses below 0.15 mm and diameters ordimensions smaller than 4 mm can be produced in high quality in a simpleway by means of the above-described process in which laser light havingwavelengths of less than 600 nm is utilized for welding thin-wallednonferrous metal sheets, without complicated after-working. The use offocus diameters of the laser beam of less than 400 pm in the continuouswelding ensures a sufficiently small heat influence zone relative to thedimensions of the hollow profile, so that no detachment of materialoccurs and a welding seam which does not have a pronounced bead on theinside of the tube is produced. Owing to the direct production of thehollow profile from nonferrous metal strips having a small wallthickness, subsequent drawing of the tube can be dispensed with.

In experiments without a drawing process subsequent to welding, coppertubes having a Ø of 2.0 mm and a wall thickness of 0.10 mm were producedat welding speeds of greater than 6 m/min by means of theabove-described process, with the welding seam quality being able to bekept constant over a number of hours.

The high welding seam quality resulting from the process of theinvention makes possible tensile stresses perpendicular to the weldingseam which is equal to the strength values of the base material. Thisallows application of pressure of the same magnitude as in the case ofseamless stress-free annealed tubes of identical diameters and wallthicknesses.

BRIEF DESCRIPTION OF THE DRAWING:

The invention will be illustrated below by way of example with the aidof an embodiment with reference to the accompanying figures. All figuresare purely schematic and not true to scale. The figures show:

FIG. 1 an illustrative example of the process of the invention for thecontinuous production of thin-walled, radially closed hollow profileswhich are corrugated in sections,

FIG. 2 an illustrative example of an apparatus according to theinvention for the continuous production of thin-walled, radially closedhollow profiles which are corrugated in sections,

FIG. 3 pictures of a welding seam of a hollow profile produced by theprocess of the invention,

FIG. 4 a schematic depiction of a process known from the prior art forreducing the wall thickness and the diameter of a tube,

FIG. 5 a schematic depiction of a tube which is corrugated in sectionsand

FIG. 6 an illustrative schematic depiction of a heat exchangercomprising a tube which is corrugated in sections and has been producedby the process of the invention.

Identical or similar elements are denoted by identical or similarreference numerals in the figures.

DETAILED DESCRIPTION:

FIG. 1 shows steps of a process 100 for producing thin-walled, radiallyclosed hollow profiles which are corrugated in sections according to oneaspect of the invention. In step 102 of the process a flat stripcomposed of nonferrous metal is fed at a first supply speed to a formingapparatus. For example, a flat copper strip is rolled off from a coil.In the forming apparatus, the flat strip supplied is formed in step 108to give a shape corresponding to the desired hollow profile, for examplea round tube. Forming can, for example, be carried out by means of aroll-forming tool.

Before forming, an optional step 106 in which one or both edges of thestrip of nonferrous metal are cut or prepared in another way can becarried out in a cutting device. In this way, the width of the strip canbe set uniformly and precisely even in the case of poor edge quality ofthe strip of nonferrous metal and the edges can optionally be preparedfor the subsequent welding operation.

The cutting device can be supplied with measured values from ameasurement apparatus which determines the width of the nonferrous metalstrip after cutting to size.

In the forming operation, the edges of the strip are conveyed by meansof guide elements so that twisting before welding is prevented and theflush adjacent edges are conveyed in a defined position and at a definedspacing past a welding apparatus. The guide elements can, for example,comprise one or more guide swords and guide bushings which are matchedto the hollow geometry to be manufactured. The closing of the geometrycan, for example, be carried out by means of drawing dies.

After forming, two opposite edges of the flat strip are located flushagainst one another in a contact region. In step 110, the flushadjoining edges in the contact region are welded to one anothercontinuously. Welding is carried out by means of a laser which emitslight having a wavelength of less than 600 nm. Blanketing of the weldingseam by protective gas can optionally be carried out from the outsideand/or inside of the hollow profile, depending on the required weldingseam quality.

After welding, the now radially closed hollow profile is taken off fromthe welding region, step 114, and in step 122 selectively corrugated insections by means of a corrugator before being fed to an uptake devicefor accommodation in step 124. Taking-off is effected by means of atransport device, for example by means of a clamping tong offtake, cleatofftake or belt offtake.

To monitor the quality of the welding seam, the temperature profiletransverse to the welding seam can be determined in an optional step112. The temperature profile determined can be supplied to a controldevice for the laser and other elements of an apparatus implementing theprocess, in particular one or more drives which regulate the supplyspeed of the strip of nonferrous metal or the speed at which the weldedhollow profile is taken off from the welding region.

The process can optionally also comprise a determination of the tensileforce on the strip before forming, step 104, and/or on the hollowprofile after welding, step 120. The tensile force determined canlikewise be supplied to the one or more drives as measured parameter forregulation.

The process can additionally comprise an optional step 116 in which oneor more dimensions of the welded hollow profile are determined. Thedimensions determined can first and foremost be supplied as inputvariables for regulating the forming operation and the cutting operationfor adjusting the width of the strip.

The process can additionally comprise an optional step 118 in which thequality of the welded seam and/or the welded material is checked formaterial defects in a non-destructive manner, for example by means ofeddy current testing, ultrasound or X-rays.

FIG. 2 shows an illustrative example of an apparatus according to theinvention for the continuous production of thin-walled, radially closedhollow profiles which are corrugated in sections. A thin strip 204 ofnonferrous metal, for example a copper strip, is rolled off from a rollor unwinder 202. The strip 204 is fed to a roll-forming tool 212 bymeans of which it is brought to the shape of the desired hollow profile,for example shaped to give a longitudinally slitted round orquadrilateral tube. A cutting apparatus 208 which cuts the strip 204 toa required width or cuts one or both edges of the strip 204 to giveclean and smooth edges can be provided between the roll or unwinder 202and the roll-forming tool 212. An uptake apparatus 205 can be providedfor accommodating offcuts of the strip 204. The width of the strip 204which has been cut to size can be checked in a strip width measurementapparatus 210. The measured results can be supplied to the cuttingdevice 208 for the purposes of regulation. In addition, a measurementapparatus 206 for determining the tensile force, the measured valuesfrom which can, for example, be used for regulating drives of theapparatus, can be arranged between the roll or unwinder 202 and theroll-forming tool 212. The edges of the strip located next to oneanother after forming of the hollow profile can be conveyed by means ofone or more guide elements 214 before the laser welding apparatus 216 insuch a way that twisting of the hollow profile before welding isprevented and the distance underneath optics of the laser weldingapparatus 216 is adhered to. The guide elements can comprise one or moreguide swords and guide bushings matched to the hollow profile. Thegeometry of the hollow profile to be welded is closed by means ofdrawing dies or guide bushings 218, so that the edges of the strip 204which has been shaped to give the hollow profile rest against oneanother in the region of the laser welding apparatus 216. The laserwelding apparatus 216 emits high-energy light at a wavelength of lessthan 600 nm, preferably in a range from 550 to 450 nm. Wavelengths in arange below 450 nm can also be advantageously used according to theinvention. The welding region can be blanketed with a protective gas,for example argon, within or outside the hollow profile via a protectivegas facility, which is not shown in the figure, in order to preventreactions of the material being welded with the atmosphere. The advanceof the welded hollow profile 224 is effected by means of a transportdevice 219. The transport device 219 can, for example, comprise one ormore clamping tong offtakes, cleat offtakes, disc offtakes or beltofftakes, or combinations thereof. After taking off from the weldingregion, one or more dimensions of the hollow profile 224 can bedetermined by means of a measurement instrument 220, preferably in acontactless manner, before the welded hollow profile 224 is fed to acorrugator 225 and then to a winder 226. To determine the tensile forcesacting on the hollow profile 224, a further tensile force measurementapparatus 222 can be provided before the corrugator 225 and the winder226. The corrugator 225 arranged upstream of the winder 226 has acorrugating tool 225 a and is configured for introducing a parallel orhelical corrugation into the hollow profile selectively in sections.

FIG. 3 shows pictures of a welding seam of a hollow profile produced bythe process of the invention. The hollow profile is a copper tube whichhas a diameter of 2 mm and a wall thickness of 0.1 mm and has beenformed and welded continuously at an advance speed of 6 m/m in from acopper strip. The welding position has been blanketed with argon on theinside and outside. FIG. 3a ) shows the welding seam on the outside ofthe hollow profile, which has a width in the range from 140 to 150 μm.FIG. 3b ) shows a photograph of the inside of the hollow profile, inwhich the welding seam has a width of about 242 μm. It can readily beseen that the welding seams are very uniform both on the inside and theoutside, so that after-working would not be necessary for mostapplications. A section of the tube produced by the process wassubjected to a pressure test and withstood pressures of more than 200bar.

FIG. 4, which shows a schematic depiction of a process known from theprior art for reducing the wall thickness and the diameter of a tube,has been described further above in relation to the prior art.

FIG. 5 shows a schematic depiction of a tube 500 which is corrugated insections. Tube 500 has two corrugated regions 502 between which anoncorrugated region 504 is located.

FIG. 6 shows an illustrative schematic depiction of a heat exchanger 600comprising a tube 500 which is corrugated in sections and has beenproduced by the process of the invention. The example depicted in thefigure shows a cross-flow heat exchanger, but other flow regimes such ascocurrent, countercurrent, etc., can also be realized using the tubewhich is corrugated in sections and has been produced according to theinvention. A partly corrugated tube 500 produced according to theinvention in a continuous process is laid in the dividing plane of atwo-part housing 602. Noncorrugated sections 504 are mounted in openingsin the housing wall which are matched to the diameter of the tube 500,for example in a bushing. Sealing between tube and housing can also beeffected in this noncorrugated region. Between the mounting points,corrugated tube regions 502 can be utilized to increase the areaavailable for heat exchange between a hot medium flowing through inlets604 into the housing 602 and a cold medium flowing through the tube 500.In addition, the corrugation promotes the formation of turbulences inthe flowing media, which likewise improves heat exchange. The tube 500is conveyed a number of times through the housing 602 in a plurality ofloops arranged in a serpentine manner. For this purpose, it isappropriately bent in corrugated regions 502. The medium to be cooledflows out of the housing through outlets 606. The flow direction of themedia is indicated by the arrows at the inlets and outlets of thehousing 602 and the tube 500. Efficient cooling of the medium to becooled can be achieved by the one-piece construction according to theinvention of the tube 500 with the smooth and corrugated sections,without joins between the smooth tube sections and the corrugated tubesections being necessary.

List of reference numerals  1 Tube  2 Die  3 Plug 100 Process 102 Supplyof strip 104 Determination of tensile force 106 Cutting of edges 108Shaping of hollow profile 110 Welding 112 Determination of temperatureprofile 114 Taking-off of hollow profile 116 Determination of dimensions118 Determination of quality 120 Determination of tensile force 122Corrugating 124 Feeding to uptake device 200 Apparatus 202 Roll/unwinder204 Strip of nonferrous metal 205 Uptake apparatus for cutting scrap 206Tensile force measurement apparatus 208 Cutting device 210 Strip widthmeasurement apparatus 212 Roll-forming tool 214 Guide element 216 Laserwelding apparatus 218 Drawing die/guide bushing 219 Transport device 220Measuring instrument 222 Tensile force measurement apparatus 224 Weldedhollow profile 225 Corrugator  225a Corrugating tool 226 Winder 500 Tube502 Corrugated region 504 Noncorrugated region 600 Heat exchanger 602Housing 604 Inlet 606 Outlet

1. A process for the continuous production of thin-walled, radiallyclosed hollow profiles which are composed of nonferrous metals and arecorrugated in sections, comprising: supply of a flat strip of thenonferrous metal at a first supply speed to a forming apparatus, wherethe thickness of the strip corresponds to the wall thickness of thehollow profile to be produced, continuous forming of the flat stripsupplied into a shape corresponding to the hollow profile, where twoopposite edges of the flat strip rest flush against one another in acontact region extending in the longitudinal direction of the hollowprofile after forming, continuous welding of the edges resting flushagainst one another in the contact region without prior treatment toreduce reflections, where the edges to be welded are conveyed at thefirst supply speed past a welding region which is fixed in relation toan apparatus implementing the process and a point in the welding regionis heated by means of a laser which emits light having a wavelength ofless than 600 nm and the heated point has a diameter which is less than20% of the cross-sectional dimension of the hollow profile, taking-offof the welded hollow profile from the welding region, selectiveintroduction of sections having corrugation into the welded hollowprofile and taking-up of the welded hollow profile in an uptake device.2. The process according to claim 1, wherein an inert protective gasflows around or blankets at least the welding region on the insideand/or outside during heating.
 3. The process according to claim 1,additionally comprising: cutting to size of one or two edges of the flatstrip of the nonferrous metal before forming.
 4. The process accordingto claim 3, additionally comprising: measurement of the width of thestrip of the nonferrous metal which has been cut to size before weldingand/or measurement of at least one dimension of the hollow profile afterwelding and regulation of the cut width and/or control of an apparatusfor forming as a function of the measurement result and a prescribedvalue.
 5. The process according to claim 1, additionally comprising:measurement of the temperature profile transverse to the welding seamand control of the energy introduced into the welding region as afunction of a comparison of the temperature profile with a prescribedprofile.
 6. The process according to claim 1, additionally comprising:checking of the welding seam by means of ultrasound, eddy currentmeasurement and/or X-rays.
 7. The process according to claim 1,additionally comprising: determination of the tensile force on the flatstrip of the nonferrous metal and/or the welded hollow profile andregulation of drives which supply the flat strip and/or the weldedhollow profile to forming, welding and/or taking-up in an uptakeapparatus.
 8. An apparatus for the continuous production of thin-walled,radially closed hollow profiles which are composed of nonferrous metalsand are corrugated in sections, comprising: a supply device equipped forsupplying a flat strip of the nonferrous metal, a forming apparatuswhich forms the flat strip of nonferrous metal into the profile of thehollow profile in such a way that the opposite edges of the flat stripof the nonferrous metal butt one another in a flush manner, two guidemeans which are at a distance from one another and between which theedges are held flush against one another, a welding apparatus whichwelds the edges which rest flush against one another between the guidemeans to one another, where the welding apparatus comprises a laserwhich emits light having a wavelength of less than 600 nm with an energywhich brings about local melting of the nonferrous metal on both sidesof the abutting edges, a transport device which conveys the weldedhollow profile further, a corrugator which is configured for introducinghelical or parallel corrugation into the welded hollow profileselectively in sections and an uptake device which takes up the hollowprofile.
 9. The apparatus according to claim 8, additionally comprising:a measurement apparatus arranged upstream of the forming apparatus fordetermining the tensile force acting on the strip supplied, where thetensile force determined is supplied to the control of drives of theapparatus.
 10. The apparatus according to claim 8, additionallycomprising: a measurement and/or regulating device which is arrangeddownstream of the welding apparatus and measures the tensile forceacting on the welded hollow profile, where the measured tensile force issupplied to a drive of the transport device for the purpose ofregulation.
 11. The apparatus according to claim 8, additionallycomprising: a cutting device which is arranged upstream of the formingdevice and by means of which one or both edges of the flat strip ofnonferrous metal which is supplied are cut, where the width of the cutstrip corresponds to the circumference of the hollow profile along theneutral fibre.
 12. The apparatus according to claim 11, additionallycomprising: an apparatus for accommodating cutting scrap.
 13. Theapparatus according to claim 11, additionally comprising: a measurementdevice arranged downstream of the cutting device for measuring the widthof the flat strip of nonferrous metal which has been cut to size. 14.The apparatus according to claim 8, additionally comprising: ameasurement device for determining a temperature profile transverse tothe welding seam, where the measured temperature profile is supplied tothe welding apparatus for controlling the energy given off and/or to thesupply device and/or the transport device for controlling the supplyspeed.
 15. The apparatus according to claim 8, additionally comprising:a measurement device for measuring at least one dimension of the hollowprofile after welding.
 16. The apparatus according to claim 8,additionally comprising: a measurement device for testing the weldingseam and/or testing for material defects or inhomogeneities of thematerial.